Polyphenolic plant secondary metabolites including flavonoids and lignans with a rich dietary experience have attracted attention as functional materials over the years due to their functional properties represented by their antioxidative activities, and are already commercially available as health foods. For example, quercetin (flavonoid), OTPP (flavonoid), sesamin (lignan), etc. are representative materials for health foods.
The biosynthetic pathway of flavonoids in plant cells has been studied since old times. Biosynthetic enzymes that catalyze the metabolic pathway and genes encoding the enzymes are isolated, leading to a better understanding of their molecular mechanisms.
On the other hand, knowledge is insufficient on how the plant secondary metabolites would be metabolized to exhibit their functions, after their in vivo uptake.
It is known that glycosylation of plant secondary metabolites is generally catalyzed by an enzyme belonging to the superfamily called UDP-glycosyltransferase (UGT), irrespective of types of sugars (glucose, rhamnose, glucuronic acid, galactose, etc.). Further in the studies of sesamin, the secondary metabolites are shown to be present in vivo as glucuronides via catechol metabolites. It is thus considered that the glucuronides would play a part in developing the in vivo functions of plant secondary metabolites.
It is confirmed that four monoglucuronides are present as the metabolites of quercetin in mammals (Q-3-GlcA, Q-7-GlcA, Q-3′-GlcA and Q-4′-GlcA) (Literature 1: Day, A J et al. Free Radic. Res. 35, 941-952, 2001, Literature 2: Moon, J. H. et al. Free Radical Biology & Medicine 30, 1274-1285, 2001, Literature 3: O'Leary, K. A. et al. Biochemical Pharmacology 65, 479-491, 2003, and Literature 4: van der Woude, H. et al. Chem. Res. Toxicol. 17, 1520-1530, 2004); in order to understand these functions in vivo, it is necessary to obtain a sufficient amount of compounds to examine their activities. However, any appropriate UDP-glucuronosyltransferase showing a broad substrate specificity is unknown so far, and it was actually impossible to chemically synthesize a binding site-specific reaction product.
The radix of Labiatae Scutellaria baicalensis is called skullcap or “wogon” in Japanese, and it is known that 7-glucuronides of highly antioxidative flavones are accumulated therein. According to the borderline of pharmaceuticals to non-pharmaceuticals, the radix of Scutellaria baicalensis is classified into the pharmaceuticals (Literature 5: Gao, Z. et al. Biochimica et Biophysica Acta 1472, 643-650. 1999). To date, Sb7GAT is purified from Labiatae Scutellaria baicalensis as flavone 7-glucuronosyltransferase; this enzyme acts only on flavones with substituents such as hydroxyl group at the ortho position of the 7-OH flavones (baicalein, scutellarein, etc.) but does not act on apigenin and luteolin which are the major flavones and further not on quercetin which is one of flavonols (Literature 6: Nagashima S. et al., Phytochemistry 53, 533-538, 2000). A gene corresponding to this Sb7GAT is registered in GenBank (Accession No. AB042277) but its function remains unconfirmed.
On the other hand, it is known that flavone 7-glucuronides which are more diverse than skullcap or “wogon” are accumulated in Perilla frutescens a red-leaf variety with a dietary experience (Literature 7: Yamazaki, M. et al. Phytochemistry 62, 987-998. 2003).